Nowadays, lithium ion secondary batteries are in widespread use as power sources for relatively small electronic devices such as mobile phones, video cameras, digital cameras, and notebook computers. Also, in recent years, inexpensive and highly safe large-sized lithium ion secondary batteries for electric automobiles, power tools, and nighttime electric power have been developed and look to be in greater demand in the future. The demand for performance of the lithium ion secondary batteries, which have been diversified and have been increasing in performance, has been increasing. In particular, there has been demand for improvement in power density and energy density to achieve an increase in performance, and for suppression of capacity degradation at high temperatures and low temperatures, improvement in cycle life, and further improvement in safety, to achieve high reliability.
Various attempts have been made to solve the above-mentioned problems, and improvements have been made. As an improvement means, optimization of constituent members of the lithium ion secondary battery including an active material used as a material of a positive electrode or a material of a negative electrode has been examined. An electrolyte solution has also been examined, and proposals have been made that relate to combinations and blend ratios of one or more selected from various solvents in which salts are to be dissolved, such as ethylene carbonate and propylene carbonate, which have a cyclic structure, and dimethyl carbonate and diethyl carbonate, which have a chain structure, combinations and blend ratios of one or more salts to be dissolved as various electrolytes, such as lithium hexafluorophosphate, lithium tetrafluoroborate, and lithium perchlorate, and combinations and blend ratios of additives for improving the above-mentioned characteristics, such as fluoroethylene carbonate and trans-difluoroethylene carbonate.
When such a non-aqueous electrolyte solution for a lithium ion secondary battery is used, degradation and deterioration of the electrolyte solution on the surface of electrodes can be suppressed depending on the combinations and blend ratios of the above-mentioned electrolyte solutions, electrolytes, and additives. This effect becomes a factor in significantly improving the characteristics of the lithium ion secondary battery such as performance and reliability.
Under the circumstances, Patent Literature 1 states that when a non-aqueous electrolyte solution to which at least one selected from the group consisting of lithium monofluorophosphate and lithium difluorophosphate is added as an additive is used, this additive reacts with lithium used as an electrode to form a good-quality coating at a positive electrode interface and a negative electrode interface, and these coatings suppress the contact between active materials in a charged state and an organic solvent and suppress the degradation of the non-aqueous electrolyte solution due to the contact between the active materials and the electrolyte solution, thus improving the preservation characteristic of the battery.
Various methods have been examined and developed as methods for producing a difluorophosphate such as lithium difluorophosphate described above. For example, Patent Literatures 2 to 5 describe methods for producing lithium difluorophosphate using hexafluorophosphate as a raw material.
Patent Literature 2 discloses a method for reacting a borate with lithium hexafluorophosphate, Patent Literature 3 discloses a method for reacting silicon dioxide with lithium hexafluorophosphate, and Patent Literature 4 discloses a method in which lithium difluorophosphate is produced by reacting a carbonate with lithium hexafluorophosphate in a non-aqueous solvent. However, a long time is required for the reactions in all these methods, and thus, from the viewpoint of productivity, it is difficult to say that these methods are useful.
Patent Literature 5 discloses a method in which a halide is added to lithium hexafluorophosphate and water, and then they are reacted in a non-aqueous solvent to produce lithium difluorophosphate. However, precise reaction control is required to stop the reaction at the time when lithium difluorophosphate, which is a target product, is obtained, and in many cases, a monofluorophosphate and lithium phosphate are produced as by-products due to an excessive reaction.
Patent Literature 6 discloses a method in which a difluorophosphate is produced by reacting a phosphorus oxyacid, a hexafluorophosphate, and an alkali salt in the presence of hydrogen fluoride. Although the hexafluorophosphate is used as a scavenger, plenty of water is produced as a by-product in this reaction, and therefore, in many cases, a monofluorophosphate is produced as a by-product due to an excessive reaction. Furthermore, the comparative example in this patent describes a problem in that use of no hexafluorophosphate as a scavenger significantly reduces the purity.
Also, in Patent Literatures 2 to 6, there is a problem in that production cost is high due to using lithium hexafluorophosphate as a starting raw material.
On the other hand, as a method using no lithium hexafluorophosphate as a raw material, Patent Literature 7 discloses a method in which a difluorophosphate is produced by bringing a carbonate and phosphorus oxyfluoride into contact with each other. However, phosphorus oxyfluoride to be used as the raw material is expensive and difficult to obtain.
Although Patent Literature 8 discloses a method for self-producing phosphorus oxyfluoride to be used as a raw material, it can be also said that this method is not suitable for industrial production for the reason that the raw materials used in the reaction between calcium phosphate and fluorosulfuric acid are expensive and the yield is low, for example.
Patent Literature 9 discloses a method in which lithium difluorophosphate is produced by bringing hydrogen fluoride into contact with lithium dichlorophosphate that has been synthesized by reacting phosphorous oxychloride with lithium carbonate. However, in this method, plenty of lithium chloride is produced as a by-product when lithium dichlorophosphate is synthesized, and an isolation load is large. Therefore, it is difficult to say that this method is efficient.
Moreover, methods are similarly required in which other difluorophosphates such as sodium difluorophosphate, potassium difluorophosphate, and ammonium difluorophosphate in addition to a lithium salt are efficiently produced in an industrial scale.